A number of metallocene catalysts have been developed and widely used for preparing a polystyrene having high stereoregularity or a polyolefin having good physical properties. The conventional metallocene catalysts have a sandwich type structure comprising a compound of a transition metal of Group IVb of the Periodic Table and a ligand having one or two cycloalkanedienyl groups such as cyclopentadienyls, indenyls and fluorenyls. The metallocene catalysts are usually employed with a co-catalyst in a polymerization process. An illustrative co-catalyst is an alkylaluminoxane, i.e., methylaluminoxane, which is a reaction product of an alkylaluminum compound with water. Syndiotactic or isotactic polystyrenes having stereoregularity can be produced by using the metallocene catalysts. In this area of stereotype polystyrenes, syndiotactic polystyrenes alternatively having benzene rings on the main chain of the polymers are significant in that the polymers have excellent physical properties such as heat resistance, since the polymers have a melting point (T.sub.m) of about 270.degree. C. due to more stereoregularity than amorphous isotactic polystyrenes.
European Patent Publication No. 210 615 A2 discloses a styrene polymer having stereoregularity and metallocene catalysts such as cyclopentadienyl trichlorotitanes and alkyl-substituted cyclopentadienyl trichlorotitanes. It is known that the metallocene catalysts of the European Patent have good catalyst activity, molecular weight distribution and syndiotactic index.
Japanese Patent Laid-Open Nos. 63-191811 and 03-250007 disclose a sulfur bridged metallocene catalyst. However, the catalyst has a disadvantage in that the yield of the catalyst is very low. Although various alkyl bridged metallocene catalysts are disclosed in Japanese Laid-Open Nos. 03-258812, 04-275313 and 05-105712, the metallocene catalysts are disadvantageous in low yield of the catalysts.
In order to overcome the shortcomings of the conventional metallocene catalysts, the present inventors have developed alkylene bridged binuclear metallocene (ABBM) catalysts, silylene bridged binuclear metallocene (SBBM) catalysts and alkylene-silylene bridged binuclear metallocene (A-SBBM) catalysts for preparing a syndiotactic polystyrene, method for preparing the catalysts, and a process for preparing syndiotactic polystyrene using the catalysts.